Alzheimer's disease (AD) has, thus far, resisted therapeutic approaches designed to slow disease progression. Reducing brain amyloid and its consequences is a therapeutic strategy in active development. One proposed means of limiting the damage caused by amyloid is to increase the rate of amyloid clearance using gene therapy. Neprilysin is a major amyloid degrading protease, and a prime candidate for use in anti-amyloid gene therapy. At present, the introduction of therapeutic genes to the brain has been limited to neurosurgical procedures which inject viral vectors directly into the brain. In addition to surgical risks, only small portions of the brain are impacted by these methods. Although many CNS disorders will likely benefit from this approach, the broad distribution of pathology in AD makes delivery of therapeutics by intraparenchymal injections challenging. As an alternative delivery method, we propose to evaluate the use of monocytes, cells normally found in blood, as vectors to transport therapeutic genes to the brain. There are several advantages of monocyte gene therapy. Neurodegenerative disorders cause neural injury and localized inflammation. Circulating monocytes naturally home to these sites of inflammation and concentrate in these areas. This is precisely where the therapeutic gene can be most effective. The therapy is reversible in the event an adverse reaction ensues due to the limited lifespan of the transfected monocytes. The patient's own cells may be harvested for use, reducing the possibility of immune reactions. The monocytes are generally post-mitotic and the transfection method does not integrate new DNA into the genome, reducing the risk of oncogenesis. The procedures involved would be relatively minor, involving intravenous blood withdrawal and reinfusion. In this application we intend to prove the feasibility of using monocyte gene therapy in a transgenic mouse model of amyloid deposition. We will exploit our prior efforts to understand the role of brain macrophages in clearing amyloid plaques, and our preliminary data showing the benefits of neprilysin gene therapy using transfected monocytes. We will identify the best monocyte fraction to use for monocyte gene therapy of the CNS. We will determine the kinetics of monocyte trafficking into the CNS of amyloid depositing mice to develop optimal dosing strategies. We will specify which of several possible mechanisms are active in clearing the amyloid deposits using a genetically engineered, secreted form of neprilysin. Finally, we will test several other amyloid degrading proteases to evaluate whether they may be useful candidates for monocyte gene therapy for AD. Success in the mouse model may rapidly lead to tests of similar gene therapy approaches in AD patients. Alzheimer's disease (AD) has, thus far, resisted therapeutic approaches designed to slow disease progression. Reducing brain amyloid and its consequences is a therapeutic strategy in active development. One proposed means of limiting the damage caused by amyloid is to increase the rate of amyloid clearance using gene therapy. In this application we intend to prove the feasibility of using monocytes, cells which naturally home to sites of inflammation, as carriers for gene therapy in a transgenic mouse model of amyloid deposition.